Of printing methods for a printer and the like, an inkjet printing method of discharging ink from an orifice (nozzle) to perform printing on a printing medium such as a printing sheet has widely been adopted in recent years because of a low-noise nonimpact printing method and high-density, high-speed printing operation.
A general inkjet printing apparatus comprises a driving means for driving a carrier holding an inkjet head, a convey means for conveying a printing sheet, and a control means for controlling these means. In order to generate energy for discharging ink from the nozzle of the inkjet head, some inkjet printing apparatuses pressurize ink by using an electromechanical transducer such as a piezoelectric element. Some apparatuses generate heat by emitting electromagnetic waves from a laser or the like. Some apparatuses bubble ink by heat generation. Some apparatuses bubble ink by heating a liquid by an electrothermal transducer having a heating resistor.
Of these apparatuses, an inkjet printing apparatus which discharges ink droplets by using heat energy can achieve high-resolution printing because nozzles can be arrayed at a high density. Especially, an inkjet head using an electrothermal transducer as an energy generation element can be easily downsized. Full use of this advantage can be made by applying an IC technique and microprocessing technique which have remarkably advanced and improved their reliability in the recent semiconductor manufacturing field. This facilitates high-density packaging and reduces the manufacturing cost.
In some supply means for supplying ink to an inkjet head, an inkjet head unit incorporates an ink tank having an absorber which holds ink. The ink tank is mounted on a carriage unit together with the inkjet head unit, and replaced together with the inkjet head unit.
According to another method, the ink tank and inkjet head of an inkjet head unit can be separated, and only the ink tank is replaced. If a large amount of ink is needed, the ink tank is fixed to an inkjet printing apparatus and supplies ink to the carriage unit via a flexible tube. This method is disclosed in, e.g., Japanese Patent Laid-Open Nos. 2001-71585 and 2002-19137.
Such an inkjet head comprises many nozzles and realizes high-speed printing by using these nozzles. However, a nozzle is clogged with dust contained in ink during printing and cannot discharge ink. A fiber of paper as a printing medium may enter a nozzle, failing to discharge ink.
In an inkjet method of boiling ink by a heater in a nozzle and discharging ink, the heater may degrade and fail to discharge ink upon many printing operations. For these reasons, the inkjet head is generally shorter in service life than other building components of the inkjet printing apparatus. A general inkjet head is therefore designed to be replaceable.
If the inkjet printing apparatus has a mechanism which allows the operator to replace an inkjet head while the apparatus is OFF or is ON but idle without any printing, a hollow needle for supplying ink to the ink chamber of the inkjet head communicates with outside air when the operator dismounts the inkjet head. Ink in the supply tube serving as an ink supply channel which connects the ink tank and inkjet head returns toward the ink tank.
Further, ink flows from the hollow needle of the ink tank serving as the connection point of the supply tube toward the air communication tube of the ink tank. As a result, the ink level in the air communication tube and the ink level in the supply tube become flush with each other and hold an equilibrium state.
After that, the operator mounts a new inkjet head. At this time, the supply tube is not filled with ink.
When an inkjet head is replaced or dismounted, the operator must turn on the inkjet printing apparatus and manually execute ink supply operation to the inkjet head. This is because the inkjet printing apparatus cannot recognize replacement or dismounting of the inkjet head in the power OFF state of the inkjet printing apparatus.
When the operator can mechanically dismount an inkjet head while the printing apparatus is ON but idle without any printing, power supply to the inkjet head stops, and various control signals to the inkjet head are disabled in order to prevent damage to the inkjet head by dismounting. Even if the inkjet printing apparatus is ON, the inkjet printing apparatus cannot recognize replacement or dismounting of the inkjet head.
If the operator does not manually perform the above-mentioned ink supply operation and the inkjet head is new, no ink exists in the ink chamber of the inkjet head. Only recovery operation of the inkjet head before printing operation cannot satisfactorily fill the ink chamber of the inkjet head with ink, generating a printing error.
When the inkjet head is not new but is dismounted, printing can start with ink left in the ink chamber of the inkjet head. However, air in the supply tube enters the ink chamber of the inkjet head, and the ink surface in the ink chamber of the inkjet head lowers, generating a printing error.
In general, to prevent generation of any printing error, the inkjet printing apparatus has a head replacement mode in which the inkjet head cannot be mechanically dismounted while the printing apparatus is OFF or is ON but idle without any printing, and can be replaced only by operation via an operation panel or the like by the operator. In this case, in response to operation to the head replacement mode, the inkjet printing apparatus moves the inkjet head to a position where the inkjet head can be replaced, or releases a mechanism which inhibits dismounting.
When the operator cancels the head replacement mode, the inkjet printing apparatus returns the inkjet head to an original position or resets the mechanism which inhibits dismounting the inkjet head. In response to cancellation of the head replacement mode, the inkjet printing apparatus interprets that the inkjet head is replaced or dismounted, and automatically executes ink supply operation and recovery operation. This method can avoid generation of the above-described printing error.
However, this method increases the cost of the mechanism which inhibits dismounting the inkjet head. In addition, the inkjet head cannot be replaced unless the apparatus is ON. Even if the operator does not actually dismount the inkjet head in the head replacement mode, ink supply operation and recovery operation are executed, wastefully consuming ink.
As another method of preventing any error, the presence of ink in the supply tube is detected to prevent entrance of air in the supply tube into the ink chamber of the inkjet head or control ink supply operation and recovery operation.
In this method, however, only the presence of ink in the supply tube is detected, and the detection precision is not increased by considering the presence of ink in the ink tank serving as a building component of the ink supply mechanism or ink in the ink chamber of the inkjet head. Ink supply operation and recovery operation complying with the detection result may be wastefully controlled.